The present invention relates to an electrode for a nonaqueous electrolyte battery having an electrode active material layer including a positive electrode active material, a conductive agent and a binder. More particularly, it relates to an electrode for a nonaqueous electrolyte battery having improved change/discharge characteristics such as discharge capacity and charge/discharge cycle life and the like.
Recent development in the electronic field is marvelous and size reduction and weight reduction of video cameras, liquid crystal cameras, portable phones, lap top computers, word processors, and others are taking place. As a power source for these devices, there is an increasing demand for the development of batteries with reduced size and weight and having a high energy density.
Conventionally, lead batteries and nickel cadmium batteries have been used for these electronic devices. However, these have been failing to sufficiently meet the demand for size reduction, weight reduction, and higher energy density.
As a battery that meets these demands, development of a nonaqueous electrolyte secondary battery using metal lithium or a material capable of being doped and undoped with lithium as a negative electrode is taking place, and a battery using lithium cobalt oxide (LiCcO2) as a positive electrode material is already put in practical use. Having characteristics of higher voltage and higher energy density as compared with conventional small secondary batteries, these batteries are highly expected as a driving power source for cordless devices, and a secondary battery can be fabricated with reduced size and weight as compared with the conventional batteries.
In order to achieve a further reduction of size, reduction of weight, and higher energy density, research and development of active materials and others is being eagerly carried out, and lithium nickel composite oxide LiNiO2 is proposed as a positive electrode active material.
Here, in an electrode for a nonaqueous electrolyte battery, a conductive agent is used because of poor electric conductivity of active materials except for some of these.
For example, Japanese Laid-open Patent Publication No. Sho 62-15761/1987 discloses a nonaqueous electrolyte secondary battery using acetylene black as a conductive agent. Though having a large specific surface area, acetylene black is liable to assume an assembled state, so that the contact property between acetylene black and a positive electrode active material seems to be poor. For this reason, if acetylene black is used as a conductive agent, decrease in capacity is large by repeated charge/discharge operations.
If graphite is used, cycle characteristics are more easily obtained than acetylene black. However, the effect as a conductive agent will not be exhibited easily unless the amount of graphite to be used is increased, so that an electrode having a high capacity cannot be obtained. This seems to be due to that fact that, since the specific surface area of graphite is small, the contact surface between the conductive agent and the active material does not increase unless graphite is used in a large amount. For example, Japanese Laid-open Patent Publication No. Hel 1-105459/1989 discloses a nonaqueous z, electrolyte secondary battery comprising a positive electrode mainly made of LiMn2O4 and graphite, a negative electrode and a nonaqueous electrolyte, wherein the amount of graphite in the total amount of LiMn2O4 and graphite is 8 to 22% by weight. This means that, in order to use graphite as a conductive agent, the effect will not be exhibited unless graphite is added in a large amount.
Further, Japanese Laid-open Patent Publication No. Hei 4-215252/1992 discloses use of a scaly graphite as a conductive agent for a positive electrode in a nonaqueous electrolyte secondary battery.
Generally, the larger the amount of the conductive agent is, the more easily the performance of the active material can be drawn out. However, if a large amount of conductive agent is put into an electrode, the mass of active material per unit volume decreases, thereby leading to decrease in the capacity as a battery.
Further, the capacity of nonaqueous electrolyte secondary batteries decreases with repeated use. One of the factors for this deterioration of the secondary batteries seems to be due to the fact that electricity cannot be taken out to the outside because contact property between the active material and the conductive agent in the electrode becomes poor.
Therefore, it is desired to maintain a good contact property between the active material and the conductive agent in the electrode while reducing the amount of the conductive agent.
Thus, in view of the aforesaid problems of the prior art, an object of the present invention is to provide an electrode for a nonaqueous electrolyte battery with improved charge/discharge characteristics such as discharge capacity and charge/discharge cycle life and the like.
The present inventors have made eager studies and found out that, with the use of crushed expanded graphite as a conductive agent, the performance of the active material can be drawn out with a smaller amount of the conductive agent, thereby completing the present invention.
Namely, the present invention is an electrode for a nonaqueous electrolyte battery, having an electrode active material layer including at least a positive electrode active material, a conductive agent and a binder, wherein at least a part of said conductive agent is a crushed expanded graphite.
In the present invention, the crushed expanded graphite preferably has a median particle diameter of 0.1 to 40 am.
In the present invention, the quantity of the conductive agent to be used in the electrode active material layer is preferably 0.1 to 15% by weight.
In the present invention, the positive electrode active material is, for example, a lithium composite oxide selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4 and LixNiyMzO2 (where x satisfies 0.8 less than x less than 1.5, y+z satisfies 0.8 less than y+z less than 1.2, and z satisfies 0xe2x89xa6z less than 0.35; and M represents at least one kind of an element selected from Co, Mg, Ca, Sr, Al, Mn and Fe).